Snow has been limited this season so far but many areas were treated to quite the ice formation show recently. In this week's Weatherwise, rime ice and surface hoar frost have both created miracles of ice, but are very different than each other.
Parts of Montana have been through a winter wonderland recently as both rime ice and surface hoar frost have encased many objects in a feathery, crystaly ice. Both deposit crystals formed by either direct freezing or sublimation on objects, usually smaller items that are freely exposed to air such as tree branches, plants, wires and power poles to name a few.
While the results are very similar, the difference between hoar frost and rime ice lies in the source of the moisture.
Rime ice moisture comes from freezing fog water droplets that turn directly from a liquid state to a solid state. Think of being in a cloud or thick fog, that moisture freezes on contact.
Surface hoar frost, meaning "grayish white", forms on clear, cold nights without fog. water vapor and moisture that's invisible above the immediate surface of the ground freezes into ice crystals. This invisible moisture sublimates, meaning it transitions directly from a gaseous state to a solid state of ice.
Areas in Montana that have been under inversions that trap moisture near the surface have experienced both, and the results have been beautiful winter scenery that is a little harder to come by so far this season.